Heavy-Duty Natural Gas Engines

Hannu Jääskeläinen

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Abstract: Interest in natural gas for heavy-duty applications can be driven by lower operating costs, tailpipe CO2 emission requirements, or ultra-low NOx emission standards. Premixed charge SI engines are very successful commercially due to their ability to achieve low emissions with relatively simple aftertreatment. However, their power and torque can be limited by knock and pre-ignition. Higher power density can be achieved in the high pressure direct injection (HPDI) approach—similar to the conventional diesel engine—but the technology is complex and may require expensive, diesel-style aftertreatment. A third natural gas technology are dual fuel engines, fueled by premixed natural gas ignited by a diesel pilot.

Introduction

Interest in natural gas for heavy-duty applications is driven by such factors as the potential for lower operating costs, compliance with tailpipe CO2 emission requirements, and as a cost effective solution to ultra-low NOx emissions.

Natural gas fuel costs are typically lower than for diesel fuel. If sufficient incentives are available to offset the higher capital cost of NG vehicles, or if the price differential between diesel and natural gas is sufficient for the purchasers of new vehicles to have a payback (usually in less than 2 years), natural gas vehicles become interesting for vehicle operators.

For manufacturers, the potential for lower tank-to-wheel CO2 emissions is an important motivator to market natural gas engines, Figure 1 [4515]. The incentive is even more significant if the well-to-tank methane emissions (methane losses) is not a significant disincentive.

[SVG image]
Figure 1. CO2 equivalent emissions (including CH4 multiplied by its GWP) for three Euro VI LNG heavy-duty trucks

IV162= Iveco Stralis Hi-road Euro VI 400hp. Spark ignited stoichiometric engine.
SC163= Scania G340 Euro VI 340hp. Spark ignited stoichiometric engine.
VO180= Volvo FH420 Euro VI 420hp. HPDI engine.
Diesel: 5 Euro VI diesels.

For achieving ultra-low NOx emissions such as California’s optional 0.02 g/bhp-hr, stoichiometric natural gas fueled SI engines have an advantage in that they can do so with relatively straightforward emission control system based on a three way catalyst (TWC). The downside is that these engines have a relatively low brake thermal efficiency (BTE) less than 39%. While diesel engines can achieve much better BTE, they require adding complexity to an already complex and expensive aftertreatment system through measures such as dual dosing urea SCR systems.

While natural gas engines are often claimed to have the potential for lower NOx and PM emissions, some have pointed out that several Euro VI natural gas heavy-duty trucks actually have higher NOx than their diesel counterparts [4521]. Natural gas vehicle proponents argue that this comparison should not be extended to tarnish all natural gas trucks [4522]. It should be noted that any difference in NOx and PM emissions between OEM natural gas and diesel vehicles is entirely attributable to differences in engine calibration and hardware choices rather than any inherent characteristics of the fuels. Vehicles using either fuel would still need to comply with applicable emission regulations. Manufacturers make engine hardware and calibration decisions based on numerous factors including available technologies, cost and expected sales volumes; the engines will still need to comply with applicable regulatory requirements. There is no incentive to over-comply for any regulatory requirements—especially if doing so would add cost and/or potentially reduce engine efficiency.

Spark Ignition Engines

Overview

Premixed charge spark ignition engines are very successful commercially due to their ability to achieve low emissions with relatively simple aftertreatment systems. Stoichiometric versions can achieve very low emissions of NOx while keeping methane emissions low with a TWC. Their relatively low thermal efficiency has been less of a concern in heavy-duty applications because the price of natural gas relative to diesel fuel has traditionally been low enough that significant fuel cost savings are still possible.

Lean burn SI engines, such as Doosan’s 11 L GL11K, have also been produced and typically use urea SCR aftertreatment to achieve low NOx emissions [4325]. However, control of methane emissions from lean burn engines is challenging and a viable commercial solution for Euro VI and EPA Phase 1 methane limits is not yet available. Thus new lean burn SI natural gas engines are no longer commonly produced for the North American and European heavy-duty markets where relatively low limits on methane emissions exist.

Table 1 illustrates some details of two stoichiometric 12L natural gas engines and a 12 L diesel engine, all certified to meet 2017 EPA CO2 limits [4467][4468][4469][4077][4177][3704]. The ISX12G natural gas engine meets 2010 NOx limits and generate CO2 credits over both FTP and SET cycles. The ISX12N natural gas engine meets CARB’s optional 0.02 g/bhp-hr limit and also generate CO2 credits over both FTP and SET cycles. The X12 diesel engine meets 2010 NOx limits and generate CO2 credits over the FTP cycle but requires credits for the SET cycle. One obvious observation is that natural gas offers the potential for significantly lower NOx and CO2 with lower technical complexity compared to the diesel engine. However, challenges remain including a lower efficiency, higher methane emissions and lower power/torque density.

Table 1
12 L stoichiometric SI natural gas engine compared to a 12 L diesel engine
Cummins ISX12 GCummins ISX12NCummins X12
FuelNatural GasNatural GasDiesel
Power/Torque1450 ft-lb@1200 rpm
400 hp @ 1800 rpm
1450 ft-lb@1200 rpm
400 hp @ 1800 rpm
1700 ft-lb@1000 rpm
500 hp @ 1761 rpm
Emissions, g/bhp-hr
FTP/SET
NOx: 0.15/0.03
PM: 0.003/0.001
NOx: 0.01/0.000
PM: 0.001/0.000
NOx: 0.17/0.16
PM: 0.004/0.003
CO2, g/bhp-hrFTP: 506
SET: 427
FTP: 502
SET: 429
FTP: 509
SET: 465
Methane emissions, g/bhp-hr1.060.190.02
N2O emissions, g/bhp-hr0.030.020.09
NH3 emissions, ppm
FTP/RMC-SET
75/160341/25
Peak efficiency (BTE)~39%1~39%144%2
AftertreatmentTWCTWCDOC/DPF/SCR/AMOX
Fuel systemThrottle body injectionThrottle body injectionDiesel common rail
1 [4510]
2 Estimated
3 For the 320 hp version [3704]

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